Abstract3‐Aryl‐4‐(5‐aryl‐Δ2‐1,2,4‐oxadiazolin‐3‐yl)sydnones (5) were synthesized in high yields by the reaction of 3‐arylsydnone‐4‐carboxamide oximes (prepared from the corresponding 3‐arylsydnone‐4‐carbonitriles) with aromatic aldehydes in the presence of acid catalysts. No reaction occurred when aliphatic aldehydes were used. The oxadiazolin‐3‐ylsydnones (5) were easily converted into the corresponding 3‐aryl‐4‐(5‐aryl‐1,2,4‐oxadiazol‐3‐yl)sydnones byN‐bromosuccinimide oxidation. The 3‐arylsydnone‐4‐carbonitrile oxides were synthesized in good yields byN‐bromosuccinimide oxidatio